In a practically applied battery, a lithium ion secondary battery has a highest energy density and has been widely used for, in particularly, small sized electronic devices. Also, in addition to a small sized usage, it has been prospected for expanding usage for vehicles. In this matter, it has been desired long term durability and more improvement of safety for a lithium ion secondary battery.
A lithium ion secondary battery normally comprises a positive electrode and a negative electrode including electrode composite material layer supported on a collector, a separator and a nonaqueous electrolyte. The electrode composite material layer comprises an electrode active material having about 5 to 50 μm of an average particle size and a binder. The electrodes are produced by forming an electrode composite material layer by coating mixture material slurry including a powdery electrode active material on the collector. Also, as for the separator for separating the positive and negative electrodes, a very thin separator having about 10 to 50 μm thickness is used. The lithium ion secondary battery is produced stacking the electrodes and the separator, and cutting them to be a predetermined electrode shape and the like. However, during this continuing producing process, the active material sometimes disengages from the electrode composite material layer and apart of the disengaged active material is included in the battery as a contamination.
The contamination like this has 5 to 50 μm of particle diameter which is about same level of the thickness of the separator, and the contamination penetrates the separator in the battery which causes a problem including short circuit. Also, heat is generated when the battery operating. As a result, a separator composed of a stretched polyethylene and the like are also heated. The separator composed of the stretched polyethylene and the like tends to shrink even though at a temperature of 150° C. or less in general, which could easily to induce a short circuit of the battery. Also, when a projection having sharp shape like a nail penetrates (for example, nail penetration tests), short circuit reaction heat is generated quickly, and a short circuit portion is enlarged.
Therefore, in order to solve such the problems, it is proposed to provide a porous protection film on a surface of the electrode. By providing the porous protection film, the disengagement of the active material at the time of producing process of the battery is prevented, and the short circuit when operating the battery is also prevented. Further, because the protection film is porous, there is no inhibition of battery reaction by penetrating an electrolyte solution in the protection film.
For example, in Patent Document 1, a porous protection film formed by using fine particle slurry including polyvinylidene fluoride as a binder and a fine particle such as alumina, silica, polyethylene resin and the like is disclosed. In Patent Document 2, a porous protection film composed thermal crosslinking resin such as polyacrylonitrile as for a binder is disclosed.
However, in case that polymer such as polyvinylidene fluoride and thermal crosslinking resin are used as the binder, there will be a risk to crack when winding due to poor flexibility of the porous film. Thus, it is necessary to perform electrode winding and the like with extremely slow winding speed. Further, in case that the crack is occurred when winding, there is a high possibility of firing of a battery, because disengaged agglomeration causes a short circuit.
Also, in Patent Document 3, as means for solving the above mentioned problem of flexibility, a porous protective film formed by using slurry obtained by dispersing inorganic filler in a resin binder composed of a water soluble polymer such as carboxymethyl cellulose and the like, and rubber particle composed of acrylonitrile-acrylate copolymer is disclosed. Although a significant improvement of the flexibility can be found by using the rubber particle as the binder, deterioration of the cyclic property can be found, because disengagement phenomenon (powder dropping) of a part of the porous film is occurred due to poor binding property. Further, due to insufficient dispersibility of the inorganic filler, film uniformity is poor.    Patent Document 1: Japanese Patent Laid Open No. 7-220759    Patent Document 2: Japanese Patent Laid Open No. 2005-332809    Patent Document 3: WO2005/011043